The present invention relates to a channel furnace for melting metals and alloys, and it concerns, more particularly, in such a furnace, a new type of single-coil inductor which assures both the heating and the forced circulation of the molten metal inside the channel of the furnace.
Already realized have been channel furnaces where each individual furnace comprises a melting tank, at least one heat-insulated duct which under the tank forms a loop situated in a vertical or oblique plane, and whose inner channel connects one heating inductor formed by an iron core transformer passing across the loop of the channel. Around the iron core of the transformer, this type of furnace comprises an excitation coil which constitutes the primary of the transformer. The secondary, having a single turn, is formed by the loop of liquid metal contained in the channel and being closed by the tank located above the channel.
Thus, the flow of an alternating current in the primary coil generates, by induction, a secondary alternating current in the loop of the channel, thereby producing development of heat by Joule effect.
This known arrangement of the channel furnace presents the following two disadvantages. On the one hand, the electric resistance of the secondary circuit is localized mainly in a long and narrow channel, while the molten metal contained in the tank offers a very wide cross-section to the flow of the current. It results therefrom that the electric heating power is released mainly in the channel and not in the tank, causing overheating of the metal contained in the channel. On the other hand, the electrodynamic stresses due to the flow of the heating current are perpendicular to the direction of this current and are exerted in a direction transverse to the longitudinal direction of the channel, thus producing turbulent movements which are undesirable as they cause wear of the refractory walls of the channel.
To remedy these disadvantages, it has been proposed to arrange an additional inductor inside or outside the loop of the channel. This inductor is formed by a polyphase winding generating a rotating field capable of producing a forced circulation of the molten metal inside the channel. It will be noted that, with a single heating inductor coil, one could already observe a certain circulation of molten metal inside the channel, but the origin of this circulation, which is generally very slow, was not well known. It was generally supposed that this very slow circulation was due mainly to the inevitable assymmetries of construction of the channel and to the secondary effects of the electrodynamic stresses due to the flow of the heating current. In any event, this very slow circulation was not sufficient to remedy the above-mentioned two disadvantages, so that it proved necessary to provide, as indicated above, an additional polyphase inductor to produce a relatively rapid forced circulation of the molten metal inside the channel.
However, when the additional polyphase inductor is arranged with the heating inductor between the core of the transformer and the loop of the channel as is generally the case, there results the disadvantage that one of the two field magnets which is situated between the other inductor and the channel forms an electromagnetic shield which reduces the magnetic coupling of the other inductor with the channel. This increases the active and reactive electrical losses and reduces the efficiency of the furnace. Besides, since the space available for accommodating the inductor or inductors inside the loop of the channel is relatively small, the fact of providing an additional inductor necessitates reducing the size of the inductor provided for the heating, thus entailing a reduction of the available heating power, and consequently of the capacity of the furnace, unless the overall dimensions of the latter are increased.